Liquid crystal displays (LCDs) have become ubiquitous in all manner of electronic gear ranging from wristwatches to large-screen televisions. First introduced in the early 1970's, LCDs have become a low cost commodity for providing high resolution, full color displays. LCDs now excel in resolution, brightness, compactness, and cost. However, in spite of nearly forty years of development, LCDs may be inadequate with respect to daylight readability and off-axis contrast and color rendition. In particular, off-axis contrast and color rendition have received a great deal of attention as display size has grown. With small displays intended for personal use, such as cell phones, calculators, and wrist-watches, off-axis contrast may not be a major issue. Larger format displays, however, may be less forgiving than the above smaller displays.
Early developers of LCD devices employed back lights to generate nearly collimated light, diffusing the light toward the viewer downstream of the Liquid Crystal (LC) panel. Since the LC panel deals only with on-axis light, there may be less loss of contrast or color rendition as the viewer moves off-axis. Light supplied to the backside of the LCD panel transits the panel in the “on-pixels” and is blocked by the “off-pixels” creating an image viewable on the front side. Light supplied in a collimated fashion may utilize a separate diffuser placed between the LCD panel and the viewer to spread the light into a viewing space.
In spite of this early demonstration of LCD technology for televisions, for the remainder of the 20th century LCD displays appeared in many small devices intended for personal use. In these applications, more emphasis was placed on compactness than on view angle performance. Back lighting solutions were adopted to provide high efficiency for mobile applications and to make the back light as thin and light as possible. Since thin back lights generally cannot accommodate conventional collimating optics, display makers generally adopted uncollimated backlights where light is emitted in a Lambertian pattern. Power consumption and portability generally won out over high viewing angle performance.
LCD panels have been included in devices aimed at the high-definition TV (HDTV) market. As this trend evolved, set makers continued to employ uncollimated back lights. Some TV manufacturers addressed the off-axis contrast and color problem by using compensation films and LCD cell architectures such as vertical alignment and in-plane switching. These approaches have helped improve viewability of modern LCD displays, but may entail certain trade-offs in complexity and/or cost. Higher end consumer LCD HDTVs employ much more sophisticated LC cell technology and polarization management layers than earlier generation devices based on twisted-nematic (TN) design, and generally have much higher cost.
The advent of HDTV has imposed some new requirements on the TV, namely its ability to provide high resolution images of 1 megapixels (720 p) and 2 megapixels (1080 p), and at large diagonals (e.g., 50″). A panel including full color pixels about 300-500 μm in size may utilize subpixels in the range of 100 μm.